The staff of a nursery school located 300 meters from a US base in Okinawa has found an object, which apparently fell on the roof from a US helicopter. Some 50 children were playing in the grounds at the time of the flyover.

The object was discovered on the roof of the Midorigaoka nursery in Ginowan City, which is located beside the US Marine Corps Futenma Air Station. The staff found it on Thursday at around 10:20am, just after a US military aircraft flew over the daycare facility, reports NHK.

Object likely from US plane at Okinawa nursery #nhk_world_newshttps://t.co/Bo6onv5cb3

Head of the nursery, Takehiro Kamiya, said staff went to check what had happened when they heard a loud sound from the roof. They found a cylindrical object that was 10cm long and 7cm in diameter. It had a warning sign that read: “Remove before flight” painted in English, according to footage shown by the TV channel.

The director said the nursery provides daycare for 61 children. Around 50 of them were playing in the garden outside at the time of the incident, while others were inside. Nobody was injured, but people working at Midorigaoka say the outcome could easily have been different.

“I'm appalled to think what would have happened if the falling point had shifted just a little bit,” said Take Nago, the 78-year-old chief caregiver at the facility, as cited by Kyodo news agency.

According to the Japanese Defense Ministry, a Sikorsky CH-53 Sea Stallion flew over Ginowan City at the time of the incident. The transport helicopters stationed at the Futenma Air Station have been a target for complaints by residents of the city, who have cited concerns about noise and risks associated with their flights. One of the most memorable incidents involving the aircraft happened in August 2004, when one of the Marine helicopters crashed into a building at Okinawa International University. Fortunately, it was largely vacant for summer break at the time. The crash was caused by poor maintenance of the helicopter, an investigation later found.

Another CH-53 crash-landed in northern Okinawa during a training exercise in October. The crash site was only a few hundred meters from the residential area of Higashi village. And last week, an F-35A Lightning II stationed at Okinawa's Kadena Airbase dropped a panel during its flight, leaving a dent in its high-tech stealth coating.